GB2342966A

GB2342966A - Piston and cylinder assembly for a disc brake

Info

Publication number: GB2342966A
Application number: GB9917944A
Authority: GB
Inventors: Kenneth James Bunker
Original assignee: Federal Mogul Technology Ltd
Current assignee: Federal Mogul Technology Ltd
Priority date: 1998-08-15
Filing date: 1999-08-02
Publication date: 2000-04-26
Anticipated expiration: 2019-08-02
Also published as: GB9917944D0; WO2000009916A1; AU5182899A; GB2342966B; GB9817767D0

Abstract

A disc brake 10 comprises discs 12, 14 mounted on a hub 16 for rotation therewith and axial sliding movement thereon, friction material pads 26 arranged on opposite sides of the discs 12, 14, and a hydraulically-operated piston 50 and cylinder 52 assembly. Egress of hydraulic fluid from the cylinder is prevented by a seal acting between the cylinder 52 and the piston 50 and comprising a stretched ring 60 of PTFE extending around said piston 50 and an elastomeric ring 64 which urges the ring 60 into contact with the piston.

Description

2342966 1 DISC BRAKE This invention is concerned with a disc brake, f or

example for a wheel of a vehicle.

A conventional disc brake comprises a disc which is mounted on a hub which provides a mounting f or a wheel. The hub is mounted, eg on a suspension link of a vehicle, for rotation about a central axis of the hub. The disc is f ixedly mounted on the hub for rotation therewith. The brake also comprises friction material pads arranged on opposite sides of the disc, and a hydraulic piston and cylinder assembly operable to urge the pads into engagement with the disc, to brake the hub and hence the wheel. Conventionally, the piston and cylinder assembly is slidably mounted and a caliper bridging an edge of the disc is fixed to the cylinder of the assembly. One friction pad is acted on directly by the piston and cylinder assembly while the other pad is mounted on the caliper on the opposite side of the disc. operation of the assembly presses one pad against the disc and causes sliding movement of the cylinder of the assembly and of the caliper to bring the other pad into engagement with the disc, thereby applying a braking force to the disc and hence to the hub.

The hydraulic piston and cylinder assembly of a conventional disc brake, as described above, comprises a seal which acts between the cylinder and the piston to prevent egress of hydraulic fluid from the cylinder. This seal is provided by an elastomeric sealing ring which is mounted in an annular groove formed in the cylinder wall, the ring projecting from the groove to engage the piston.

2 This ring also serves as an energy storage mechanism. Specifically, when the assembly is operated to move the piston outwardly of the cylinder to put the brake "on", the ring is compressed thereby storing energy therein. When the pressure of the hydraulic f luid in the cylinder is reduced, the ring releases the stored energy therein by moving the piston inwardly of the cylinder (away from the disc). Movement of the piston away from the disc (retraction of the piston) allows the pads of the brake to move away from the disc putting the brake into a "brakes off" condition. Accordingly, the sealing ring has to have a complex structure and also has to engage the piston with a significant force which makes movement of the piston more difficult and accordingly increases the energy required to operate the brake.

US 4809821 A discloses a conventional disc brake having a PTFE ring covering lands of a cylinder to prevent corrosion, there being a separate seal. US 4382492 A discloses a conventional disc brake having a low friction seal but also having a separate retraction assembly with a plastic ring which moves with the piston. US 4336746 A discloses a retraction unit for the piston of a disc brake, the unit comprising compressible rings which may be of PTFE.

In WO 98/25804, a disc brake is described in which two discs are mounted on the same hub. Each disc is mounted on the hub by mounting means which is arranged so that the hub and the discs rotate about the central axis of the hub as a unit but the discs can perform axial sliding movement on the hub. The brake also comprises a piston and cylinder assembly having its cylinder f ixed relative to a suspension link of a vehicle, the cylinder being integrally formed with the link. The brake also comprises a caliper fixed relative to the cylinder, the caliper providing supports for friction pads of the brake, including pads between the 3 two discs. The pad furthest from the assembly is fixed to the caliper but the remaining pads are slidably mounted thereon. When the piston and cylinder assembly is operated, the pad operated upon by the assembly and the pads between the discs slide on the caliper, and the discs slide on the hub until each disc is engaged on both sides by a pad.

It is an object of the present invention to provide a disc brake in which the amount of energy required to operate the brake is reduced.

The invention provides a disc brake comprising a disc which is mounted on a hub so that the hub and the disc rotate about a central axis of the hub but the disc can perform axial sliding movement on the hub, the brake also comprising friction material pads arranged on opposite sides of the disc, and a hydraulically-operated piston and cylinder assembly which is operable to move a piston thereof out of the cylinder thereby moving a first of the pads into contact with the disc and causing movement of the disc to bring the other pad into contact with the disc so that the pads apply a braking force to the disc, the brake also comprising a seal acting between the cylinder and the piston to prevent egress of hydraulic fluid from the cylinder, wherein said seal comprises a ring of low friction material extending around said piston in sealing engagement therewith, and an elastomeric ring which encircles said ring of low friction material and resiliently urges it into contact with said piston, said elastomeric ring being in sealing engagement with said ring of low friction material and with said cylinder.

A disc brake according to the invention has a low friction sliding seal between the sealing ring and the piston so that less energy is required to operate the brake. This is made possible because, in a brake according 4 to the invention, the sealing ring is not utilised to store energy for moving the piston relative to the cylinder neither is there a separate retraction unit. Instead, when the brake is to be put into a "brakes off" condition, the disc moves rather than the caliper pushing one friction pad and the piston inwardly of the cylinder. This movement of the disc and consequent retraction of the piston is caused by the brake pads and the disc creating separation forces by their contact when the cylinder is not pressurised.

A disc brake according to the invention may be generally as disclosed in WO 98/25804 but may have other forms provided that the disc or discs are slidable axially on the hub.

In a disc brake according to the invention, the sealing ring of low friction material may be in tension to improve sealing.

The low friction material may comprise polytetrafluoroethylene or other low friction material. The polytetrafluoroethylene ring can be formed by sintering polytetrafluoroethylene particles, eg into a cylindrical shape from which the ring can be split.

There now follows a detailed description, to be read with reference to the accompanying drawings, of a disc brake which is illustrative of the invention.

In the drawings:

Figure 1 is a vertical cross-sectional view taken through the illustrative disc brake; and Figure 2 is a detailed view of a portion of the illustrative disc brake, the portion being indicated by a circle in Figure 1.

The illustrative disc brake 10 comprises two discs 12 and 14 which are mounted on a hub 16 so that the hub and the discs rotate about a central axis 18 of the hub 16, but the discs can perform axial sliding movement on the hub 16.

The hub 16 comprises an internally splined hollow inner cylindrical portion 16a which is arranged to receive a drive shaft (not shown) which drives the hub and a wheel (not shown) mounted thereon. The hub 16 also comprises an external flange 16b at one end of the portion 16a, against which the wheel can be bolted in a conventional manner. The flange 16b also serves to connect the portion 16a to a mounting portion 16d of the hub 16. The mounting portion 16d is generally in the shape of a hollow cylinder open at one end, closed by the portion 16b at the other end, and having a cylindrical outer surface 16e.

The discs 12 and 14 are identical to one another, being in the form of general ly-annular relatively-thin cast iron or steel plates. The two discs 12 and 14 are mounted by means of mounting means of the brake 10 on the cylindrical outer surface 16e of the hub portion 16d so that the two discs 12 and 14 rotate as a unit with the hub 16 about the axis 18 and the discs 12 and 14 can perf orm axial sliding movement on said hub 16. The mounting means comprises grooves 20 which are formed in the outer cylindrical surface 16e of the portion 16d of the hub 16 and teeth 22 which project inwardly from each of the discs 12 and 14. The teeth 22 enter the grooves 20 and are a sliding fit therein.

The brake 10 also comprises four friction material pads 26 which are arranged with two of the pads 26 on opposite sides of each of the discs 12 and 14. The pads 26 are for braking the discs 12 and 14 by engaging side surfaces of the discs. The friction pads 26 are secured to three backing plates 28, one backing plate 28a being between the 6 discs 12 and 14 and the others being on opposite sides of the discs 12 and 14 to the plate 28a. The median plate 28a has friction pads 26 secured to both of its surfaces.

The brake 10 also comprises resilient means in the form of leaf springs 32 of the type described in WO 98/25804. The leaf springs 32 are mounted on the hub 16 and engage the discs 12 and 14 so that the springs 32 apply force radially between each of the discs 12 and 14 and the hub 16 to control the movement of the discs on the hub. The leaf springs 32 are distributed circumferentially around the hub 16, each being mounted in one of the grooves 20 to engage the teeth 22 which enter the grooves. The leaf springs 32 act between the teeth 22 and bottom surfaces 20a of the grooves 20.

The brake 10 also comprises an operating mechanism in the form of a hydraulically-operated piston and cylinder assembly comprising a piston 50 and a cylinder 52, the cylinder 52 is fixed relative to the axis 18 being integral with a suspension link (not shown) of the vehicle as disclosed in WO 98/25804. The assembly 50, 52 is operable to move a first of the pads 26 into contact with the disc 12 and to cause relative movement between the assembly 50, 52 and the disc 12 to bring the other pad 26 associated with the disc 12 into contact with the disc 12, so that the pads 26 apply a braking force to the disc 12. Specifically, a bridge 54 is mounted on the cylinder 52 and bridges the edges of both discs 12 and 14. The furthest pad 26 from the cylinder 52 has its support plate 28 fixedly mounted on the bridge 54 while the support plates 28 of the remaining pads 26 are slidable on the bridge 54. When the piston 50 is moved outwardly from the cylinder 52 by hydraulic fluid under pressure, it directly presses the support plate 28 of the nearest pad 26 so that the plate 28 slides on the bridge 54. The nearest pad 26 presses the disc 12 so that it slides on the hub 16. The disc 12, in 7 turn, presses on the pad 28 on the opposite side thereof causing the support plate 28a to slide on the bridge 54. The movement of the plate 28a causes the pad 26 on the plate 28a which faces the disc 14 to contact the disc 14. The disc 14 slides on the hub 16 until it contacts the pad 26 on the plate 28 which is fixed to the bridge 54. The brake 10 is now "on" with the discs 12 and 14 both engaged on both sides by one of the pads 26. The pads 26, therefore, apply braking forces to the discs 12 and 14 which forces are transferred to the hub 16 by the teeth 22.

The illustrative brake 10 also comprises a seal 56 acting between the cylinder 52 and the piston 50 to prevent egress of hydraulic fluid from the cylinder 52. The seal 56 (see Figure 2) comprises a ring 60 which is mounted in an annular groove 62 in the wall of the cylinder 52 and projects out of the groove 62 to sealingly engage the piston 50. The ring 60 has a rectangular transverse crosssectional shape and is formed from polytetrafluoroethylene. The ring 60 is, thus, formed of low friction material.

The ring 60 is in tension having been stretched to fit it around the piston 50. The ring 60 is also resiliently urged into sealing engagement with the piston 50 by an elastomeric ring 64 which encircles the ring 60 and is contained within the groove 62. The ring 64 has a circular transverse cross-sectional shape. The ring 64 is also in sealing engagement with the ring 60 and with the bottom of the groove 62, ie with the cylinder 52.

When the brake 10 is to move into a "brakes off condition, the pressure in the cylinder 52 is released. The discs, 12 and 14, and the pads 26 then create separation forces therebetween by contacts. These separation forces cause the discs 12 and 14 and the movable pads 26 to move away from the fixed pad 26 and away from one another, ie they move to the left viewing Figure 1.

8 The piston 50 is pushed into the cylinder 52 against the low frictional force applied by the seal 56 to the piston 50.

9

Claims

A disc brake comprising a disc which is mounted on a hub so that the hub and the disc rotate about a central axis of the hub but the disc can perform axial sliding movement on the hub, the brake also comprising friction material pads arranged on opposite sides of the disc, and a hydraulically-operated piston and cylinder assembly which is operable to move a piston thereof out of the cylinder thereby moving a first of the pads into contact with the disc and causing movement of the disc to bring the other pad into contact with the disc so that the pads apply a braking force to the disc, the brake also comprising a seal acting between the cylinder and the piston to prevent egress of hydraulic fluid from the cylinder, wherein said seal comprises a ring of low friction material extending around said piston in sealing engagement therewith, and an elastomeric ring which encircles said ring of low friction material and resiliently urges it into sealing engagement with said piston, said elastomeric ring being in sealing engagement with said ring of low friction material and with said cylinder.
2 A disc brake according to Claim 1, wherein said ring of low friction material is in tension.
3 A disc brake according to either one of Claims 1 and 2, wherein said low friction material comprises polytetrafluoroethylene.
4 A disc brake substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.